Organic light-emitting devices such as described in U.S. Pat. No. 5,247,190 or in U.S. Pat. No. 4,539,507, the contents of which are incorporated herein by reference, have great potential for use in various display applications. According to one method, an OLED is fabricated by coating a glass or plastic substrate with a transparent first electrode (anode) such as indium tin oxide (ITO). At least one layer of a thin film of an electroluminescent organic material is then deposited prior to a final layer which is a film of a second electrode (cathode) which is typically a metal or alloy.
In many practical applications, the layer of electroluminescent organic material has a thickness of the order of 100 nm in order to ensure a practical operating voltage. It is typically deposited on the first electrode by a spin-coating technique. If the organic material is contaminated with particles having a size of the order of the thickness of the organic layer, not only will these particles themselves give rise to defects in the resulting organic layer, their presence disrupts the movement of the fluid organic material over the surface of the first electrode layer leading to variations in the thickness of the resulting organic layer about the particle, and in the worst case leading to the formation of holes in the organic layer through which the underlying layer (electrode layer) is exposed.
Defects in the organic layer can also be caused by, for example, inherently poor film-forming properties of the organic material, or by physical damage to the organic layer after deposition.
A typical defect site is shown in FIG. 3. The electroluminescent organic layer 106 has been deposited by spin coating on a glass substrate 102 coated with an indium tin oxide (ITO) anode layer. The existence of a large particle 107 has led to a defect site 109 comprising the particle 107 itself and a pinhole 111. A cathode layer 110 is formed over the electroluminescent organic layer 106.
Localised defects of the kind shown in FIG. 3 can manifest themselves during device operation as a current anomaly (short) where a large proportion of the current becomes localised in the area of the defect. This leads, inter alia, to problems of device reproducibility and is a particular problem in dot matrix devices since it provides alternative current paths that lead to the wrong pixels being lit.
In order to prevent these kind of defects, the deposition of the organic layer is typically carried out in a clean room with a view to preventing contamination and typically involves filtering the organic material prior to spinning to remove large particles therefrom. However, a typical clean room has particle size levels specified down to 300 nm and the organic material is only typically filtered to about 450 nm, since the elimination of particles having smaller sizes requires great expense.
The light-emissive organic material will therefore often still be contaminated with particles having a size of the order of the thickness of the organic layer to be deposited, which will, as mentioned above, lead to defects in the resulting organic layer. Furthermore, even if the contamination by such large particles could be completely eliminated, defects can still arise during the manufacturing process as a result, for example, of inherently poor film-forming properties of the organic material itself, or due to physical damage inadvertently inflicted on the organic layer after deposition.
One known technique of removing the defect particles after production of the device is by passing a very high current through the device to “burn-out” the defect particles by vaporizing them. However, this technique is not applicable to all defect particles and cannot be used to resolve the problem of large shorts. Moreover, it does not necessarily deal with problems that may manifest themselves in the lifetime of the device. It is therefore an aim of the present invention to reduce the problem of current anomalies in an organic light-emitting device.
In organic light-emissive devices (OLED's) such as those described in our earlier U.S. Pat. No. 5,247,190 or in Van Slyke et al.'s U.S. Pat. No. 4,539,507, light emission from the at least one organic layer occurs only where the cathode and the anode overlap and therefore pixelation and patterning is achieved simply by patterning the electrodes. High resolution is readily achieved and is principally limited only by the overlap area of the cathode and the anode and thus by the size of the cathode and the anode. Dot-matrix displays are commonly fabricated by arranging the cathode and the anode as perpendicular arrays of rows and columns, with the at least one organic layer being disposed therebetween.
Low resolution dot-matrix displays can, for example, be fabricated by coating at least one organic electroluminescent layer onto a substrate having thereon an array of indium-tin oxide (ITO) lines which act as an anode. A cathode comprising an array of lines perpendicular to those of the anode is provided on the other side of the at least one organic layer. These cathode lines may, for example, be lines of aluminium or an aluminium-based alloy which can be evaporated or sputtered through a physical shadow mask. However, shadow masking may not be desirable for various reasons. In particular, there are significant constraints on the use of shadow masks when displays of large area and/or high resolution are required. In order to produce such electrode line arrays and other patterns of large area and/or high resolution one would normally have to use various forms of lithography.
In order to fabricate efficient and stable OLED's with the desired electrical and light output characteristics great care must normally be taken in the design and construction of the interfaces between any organic layer and the electrodes. The particular importance of these interfaces is due to the fact that charge carriers should be injected efficiently from the electrodes into the at least one organic layer.
Maintaining the desired electrical and light output characteristic of the pixels in an OLED display when lithographic processes are used to fabricate the electrode patterns, in particular where those patterns are on top of the at least one organic layer, is not trivial owing to the risk of the lithographic processes modifying and potentially damaging the organic layer/electrode interfaces and the vicinity. Such damage during lithography may originate from the photoresist, the developers, the etching processes (both dry and wet, negative and positive techniques and etch and lift-off) or the solvents used. It should be mentioned here that conjugated polymers are often deposited from and are generally soluble in organic solvents.
Plasma etching/ashing is very often used in lithography to remove the photoresist or residual photoresist which may not have been washed off the developer. Organic electroluminescent and charge transporting materials would normally be damaged, modified and/or etched very rapidly in such dry etching/ashing processes if directly exposed to the plasma.
One method of protecting the organic electroluminescent and charge transporting materials from the effects of the electrode patterning processes is disclosed in WO97/42666 in which a thin barrier layer composed of a dielectric material is interposed between the conductive electrode layer and the layer of light-emissive organic material.
The inventors of the present invention have identified the requirement for an improved construction which allows for the use of various lithographic processes to form the electrode on top of at least one organic layer without significantly changing the electrical and light output characteristics of the display, and which meets todays demands for increased efficiency, reliability and durability. It is therefore another aim of the present invention to provide a device which meets these requirements.